If you haven't done any pet scroll quests stop lvling her and do all the quests that give pet scrolls. Use all your pet scrolls and lower her int and str as much as possible and add it to wis, agi and con instead. This is if you wish for Xao to just focus mainly on stone walling. She will need a large amount of sp for stonewalling so alot of wis unless you have a +400sp scroll for her.

If you've done all the pet scroll quests and used up most/all of them and you can't buy pet lethe scrolls then I suppose you should make her into an attacker instead. With this option she will keep adding points to int when she levels and occasionally to the other options too.

However if you can't do option A and option B doesn't appeal to you then I suggest changing your pet to another one that assists like Xaolan does or seals if you don't want an attacking pet.